1. Field of the Invention.
The present invention relates to a method for controlling the supply of fuel for an internal combustion engine.
2. Description of Background Information
Among internal combustion engines for a motor vehicle, there is a type in which fuel is supplied to the engine via a fuel injector or fuel injectors.
As an example, there are systems in which the pressure within the intake pipe downstream of the throttle valve, and the engine rotational speed (referred to as rpm (revolutions per minute) hereinafter) are sensed and a basic fuel injection time T.sub.i is determined according to the result of the detection at predetermined intervals synchronized with the engine rotation. The basic fuel injection time T.sub.i is then multiplied with a correction coefficient determined according to engine parameters such as the engine coolant temperature or a transitional change in the engine operation. In this manner, an actual fuel injection time T.sub.out corresponding to the required amount of fuel injection is calculated.
However, in this arrangement, there is inevitably a delay of control operation between a time of detection of the pressure within the intake manifold and a time of actual fuel injection. This means that the pressure in the intake manifold at the time of actual fuel injection may greately differ from the detected pressure especially when the pressure change in the intake manifold is relatively large, such as in the case of the acceleration of the engine. Therefore, a control method was proposed and described in Japanese Patent Application No. 59-104315 which was assigned to the same assignee of the present application. In this control method, the pressure in the intake manifold at the time of actual fuel injection is estimated, for example, from a manner of variation of the detected value of the pressure in the intake manifold. The amount of the basic fuel injection is determined in accordance with the estimated value of the pressure in the intake manifold.
However, during idling of the engine, the opening degree of the throttle valve is small and substantially constant. Therefore, the pressure in the intake manifold does not follow the change in the engine rotational speed especially in the case where the capacity of the intake manifold is relatively large. Therefore, the amount of the fuel injection can not be determined appropriately even though an estimation of the pressure in the intake manifold at the time of the fuel injection is performed.
In order to solve this problem, a technique is proposed in which the engine rotational speed at the time of fuel injection is estimated and the basic fuel injection amount is corrected according to the estimated value of the engine rotational speed.
In this type of method for controlling fuel supply, it is general to detect an idling range of the engine in terms of the pressure within the intake manifold and the engine rotational speed. Specifically, the idling of the engine is detected as a state in which the engine rotational speed is lower than an idling reference speed of the engine, and an absolute pressure of the intake air in the intake manifold is lower than a reference pressure for detecting the idling of the engine. The idling reference speed is set at a level slightly higher than a stable rotational speed at which the engine rotational speed becomes stable during a no-load condition of the engine after the warming-up of the engine. Also, the reference pressure is determined at an absolute pressure level which is slightly higher than an absolute pressure of the intake air which is obtained when the engine is operating at the stable rotational speed mentioned above. This is because, in the case of an engine mounted on a vehicle, the rotational speed of the engine is raised during a period in which the engine is idling while an air conditionar of the vehicle is operated.
However, if the operating condition of the engine falls in the thus defined idling range of the engine operation while the engine is decelerating, the amount of fuel supply may be changed discontinously because the method of calculation of the fuel supply amount is different between inside and outside of the idling range of the engine. This may result in a sensible change in the engine speed which causes a shock being felt by a driver or a passenger of the vehicle.